The present invention relates generally to electrochemical double layer capacitors, and more particularly to a high performance electrochemical double layer capacitor made with low-resistance carbon powder electrodes and low resistance current collectors.
Double layer capacitors, also referred to as electrochemical double layer capacitors (EDLC), are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, because of their relatively low internal resistance, double layer capacitors can typically be charged and can, in turn, deliver stored energy at a higher power rating than rechargeable batteries.
Double layer capacitors may consist of two carbon electrodes that are isolated from electrical contact by a porous separator. Both the porous separator and the electrodes are immersed in an electrolyte solution, allowing ionic current (ionic flow) to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the two carbon electrodes.
Coupled to the back of each of the two carbon electrodes is typically a current collecting plate. One purpose of the current collecting plates is to reduce ohmic losses, i.e., internal resistance, in the double layer capacitor.
Double layer capacitors store electrostatic energy in a polarized liquid layer that forms when an electrical potential exists between the two carbon electrodes immersed in an electrolyte (or electrolyte solution). When the electrical potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name xe2x80x9cdouble layerxe2x80x9d capacitor) by the polarization of electrolyte ions due to charge separation under the applied electrical potential, and also due to dipole orientation and alignment of electrolyte molecules over an entire surface of the electrodes.
Fabrication of double layer capacitors with carbon electrodes is described in U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon-electrode capacitors, including electrochemical double layer capacitors with carbon electrodes, is that the performance of the carbon-electrode capacitor is often limited because of high internal resistance related to the carbon electrodes. This high internal resistance may be due to several factors, including high contact resistance of carbon-carbon contacts within the carbon electrodes, and further including high contact resistance of the electrode-current collector contacts. This high internal resistance translates to large ohmic losses in the carbon-electrode capacitor during charging and discharging of the carbon-electrode capacitor. These high ohmic losses further adversely affect, i.e., increase, a characteristic RC (resistance times capacitance) time constant of the capacitor and thus interfere with the carbon-electrode capacitor""s ability to be efficiently charged and/or discharged in a short period of time.
There is thus a need in the art for systems and methods that lower the internal resistance within a carbon-electrode capacitor, and hence lower the characteristic RC time constant, of the carbon-electrode capacitors, as well as other improvements.
U.S. Pat. No. 5,907,472 to Farahmandi et al., the complete disclosure of which is incorporated herein by reference, discloses a multi-electrode double layer capacitor having aluminum-impregnated carbon cloth electrodes. The use of the aluminum-impregnated carbon cloth electrodes described therein results in an electrochemical double layer capacitor having a very low internal resistance.
U.S. patent application Ser. No. 10/005,885 (Attorney Docket No. 71752), filed Nov. 2, 2001, for ELECTROCHEMICAL DOUBLE LAYER CAPACITOR HAVING CARBON POWDER ELECTRODES, the complete disclosure of which is incorporated herein by reference, discloses electrochemical double layer capacitors having low-resistance carbon powder electrodes.
There is also a continuing need for improved electrochemical double layer capacitors. Such improved electrochemical double layer capacitors need to deliver large amounts of useful energy at a very high power output, and very high energy density ratings within a relatively short period of time. Such improved electrochemical double layer capacitors should also have a relatively low internal resistance, and hence a relatively low characteristic RC time constant, and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is apparent that improvements are needed in the techniques and methods of fabricating electrochemical double layer capacitors and capacitor electrodes so as to minimize the process steps of manufacturing the electrochemical double layer capacitor, and hence reduce associated time and costs. Likewise, technique improvements are needed as well to lower the internal resistance of the electrochemical double layer capacitor. Lowering internal resistance would result in lowering the characteristic RC time constant and maximize the operating voltage.
Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. Thus, improved techniques and methods are needed to both lower the internal resistance of the electrodes used within an electrochemical double layer capacitor and minimize fabrication process steps.
The present invention addresses the above and other needs.
The present invention advantageously addresses the needs above as well as other needs by providing an electrochemical double layer capacitor and a method of making an electrochemical double layer capacitor.
In accordance with one embodiment, the present invention can be characterized as an apparatus for use in a double layer capacitor. The apparatus has a can; a terminal post at a basal end of the can formed integral with the can and extending down from the exterior of the basal end of the can.
In accordance with another embodiment, the present invention can be characterized as an apparatus for exposing surfaces and closing gaps between windings at contact edges of an electrode assembly. The apparatus is made up of a disk; and turbine-like ridges extending radially from a center of the disk on one side of the disk for collecting the contact edges of the electrode assembly.
In a further embodiment, the present invention can be characterized as a method of making a double layer capacitor. The method has steps of providing a can having a terminal post at a basal end of the can formed integral with the can and extending from the exterior of the basal end of the can, the can having a terminal post at a basal end of the can formed integral with the can and extending from the exterior of the basal end of the can; providing an electrode assembly comprising activated carbon; and inserting the electrode assembly having electrode contact edges on proximal and distal ends of the electrode assembly into the can.
In another further embodiment, the present invention can be characterized as a method for exposing electrode contact edges on an end of an electrode assembly and closing gaps between windings at the electrode contact edges. The method includes holding the electrode assembly on a rotational axis; forcing an end of the electrode assembly into one surface of a collecting tool; and rotating the collecting tool and the electrode assembly relative to one another.
In yet a further embodiment, the present invention can be characterized as a method of making an apparatus for use in a double layer capacitor with the steps of providing metal for molding; and impact molding the metal to form a can, including: forming a terminal post at a basal end of the can integral with the can and extending down from an exterior of the basal end of the can.
In yet another embodiment, the present invention can be characterized as an apparatus for use in a double layer capacitor. The apparatus has a double layer capacitor electrode assembly having electrode contact edges on a proximal and on a distal end of the double layer capacitor electrode assembly; a top collector disk electrically coupled to the electrode contact edges of the proximal end of the double-layer capacitor electrode assembly; and a lid electrically coupled to the top collector disk.
In an additional embodiment, the present invention can be characterized as an apparatus for use in a double layer capacitor. The apparatus includes an electrode assembly having electrode contact edges on a distal end of the electrode assembly; a bottom collector disk at the distal end of the electrode assembly; and a can in which the double layer capacitor assembly is located and wherein a peripheral edge of the bottom collector disk contacts an interior wall of the can.
In yet an additional embodiment, the present invention can be characterized as a method of making an apparatus for use in a double layer capacitor including steps of electrically coupling a top collector disk to a proximal end of a double layer capacitor electrode assembly; heating a lid having a flange on a bottom of the lid; and placing the lid over a structure on the top collector disk, wherein the flange engages the structure on the top surface of the top collector disk.
In a supplemental embodiment, the present invention can be characterized as a method of making a double layer capacitor. The method has steps of electrically coupling a top collector disk to a proximal end of a double layer capacitor electrode assembly; electrically coupling a bottom collector disk to a distal end of the double layer capacitor electrode assembly; heating a can to increase a diameter of the can; inserting the double layer capacitor electrode assembly into the can; cooling the can to decrease the diameter of the can, wherein a peripheral edge of the bottom collector disk is coupled to an interior wall of the can as the diameter of the can is decreased; forming a bead around an exterior of the can at a location of the top collector disk; heating a lid to increase a diameter of a concave structure on the lid; placing the concave structure of the lid in juxtaposition with a convex structure on the top collector disk; cooling the lid to decrease the diameter of the convex structure, wherein the concave structure is coupled to the convex structure as the diameter of the can is decreased; creating a seal between the lid and the can; and placing an electrolyte solution into the electrode assembly.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings, which set forth an illustrative embodiment in which the principles of the invention are utilized.